One quick way to tell is:
The G-Max uses the Red flappers
The Power Gravity uses the Blue flappers.
I told my builder to order and install 5 MS854114S toto ultramax in my new house. I've checked under the lid and i don't see the GMAX, i see POWER GRAVITY.
I can't find the model number in the tank anywhere, it has these numbers but none of them look like model numbers. IS it hidden somewhere?
One piece Toto's will have the model # stamped in black inside the tank on the back. It should say CST854S for your toilet.